(1) Field of the Invention
The present invention relates to a fluorescent lamp and, in particular, to a fluorescent lamp for use for a backlight source for display devices, such as liquid crystal displays, glass for fluorescent lamps which is appropriate for fluorescent lamps, and a glass tube using the same.
(2) Related Art Statement
Liquid crystal display devices widely used in televisions and monitors have a backlight made up of optical members, such as a light source provided on the rear or side of the liquid crystal display portion, a reflective sheet for making light emitted from the light source uniform, a light guide plate, a diffusion sheet and a prism sheet, and illuminate the liquid crystal display portion from the rear, so that images can be displayed on the liquid crystal display portion.
In addition, high image quality performance is required for liquid crystal devices in terms of high brightness, high resolution and high contrast, and furthermore, reduction in the thickness, weight and power consumption, as well as prolonging of the life, in addition to increased image quality, are also required, in terms of properties to be provided. Moreover, it is necessary to take safety and the environment into consideration.
Accordingly, it is necessary for the light source of liquid crystal display devices to have certain properties, in order for the liquid crystal display device to meet these requirements, and fluorescent lamps are generally used. Though there are various types of fluorescent lamps, cold cathode type fluorescent lamps (hereinafter referred to as cold cathode fluorescent lamps) are often used, for the reason that it is possible to make the tube thin, so that reduction in the thickness of the liquid crystal display device is easy, their life is long, and the required brightness can be secured, along with the elongation of the tube.
Cold cathode fluorescent lamps have a structure where electrodes are provided at the two ends of the glass tube, an inert gas, such as argon (Ar) or neon (Ne), and mercury are sealed in the tube with relatively low pressure, and a fluorescent body is applied on the inner wall of the tube, so that discharge between the electrodes generates ultraviolet rays and the fluorescent body is stimulated mainly by exciting lines of 254 nm and 185 nm and emits light, and thus, necessary visible light can be gained.
Currently, long cold cathode fluorescent lamps having an outer diameter of approximately 3 mm, a thickness of approximately 0.5 mm and a length of 500 mm or more are m use.
In addition, conventional electrodes are formed of portions in cup form made of a metal, such as nickel, tantalum or tungsten, and lead portions which penetrate through the glass tube and connected to the portions in cup form. In addition conventional leads are formed of Kovar (alloy of Fe, Ni and Co) or tungsten, and penetrate through the glass and are sealed by the glass in the end portions of the glass tube, so that the fluorescent lamp can be kept airtight.
It is important for the glass used for cold cathode fluorescent lamps to have physical strength, because the diameter of the tube is small, and the tube is relatively thin and long.
In addition, the temperature of the glass around the electrode portions becomes high during the operation of the fluorescent lamp, and therefore, it is necessary for the coefficient of thermal expansion of the leads and the coefficient of thermal expansion of the glass to be as close as possible, in order to maintain the heat resistance, and so that the sealed portions of the leads remain airtight.
In addition, ultraviolet rays are continuously generated as a result of the discharge inside the tube during operation, and therefore, it is important for the glass to be prevented from becoming colored due to solarization by the ultraviolet rays. Furthermore, optical members provided around the fluorescent lamp are usually made of plastic, and thus, the color changes in the optical members when ultraviolet rays leak from the fluorescent lamp. Therefore, it is necessary for the glass to be provided with such properties as to block ultraviolet rays, in order to reduce the amount of ultraviolet rays that leak from the fluorescent lamp.
Concretely, ultraviolet rays generated within fluorescent lamps include exciting lines of 313 nm, in addition to the above described 185 nm and 245 nm lines, and thus, it is effective for preventing the color of optical members made of plastic from changing to block 313 nm exciting lines, in addition to 185 nm and 254 nm exciting lines.
It is necessary for the glass of cold cathode fluorescent lamps used as a light source for liquid crystal display devices to have the above described properties, and the glass which usually contains B2O3 as a main component, in order to gain physical strength and excellent heat resistance, and has glass compositions where elements having such properties as to prevent solarization and block UV rays, such as TiO2, CeO2, Fe2O3, Sb2O3 and Nb2O3, are added alone, or two or more are added, as elements have been examined.
Japanese Unexamined Utility Model Publication H5 (1993)-8861 describes a fluorescent lamp having a glass tube containing 0.01% to 0.1% of iron oxide (Fe2O3) and 0.01% to 1.0% of titanium oxide (TiO2) by weight, where the average coefficient of linear expansion α is preferably 80×10−7/° C. to 110×10−7/° C.
Japanese Patent No. 3575114 describes a glass tube using glass containing 55.0% to 73.0% of SiO2, 10.0% to 21.7% of B2O3, 1.0% to 10.0% of A2O3, 0% to 4.0% of Li2O, 0% to 3.2% of Na2O, 2.6% to 15.0% of K2O, 4.0% to 16.0% of Li2O+Na2O+K2O, 0.05% to 9.0% of TiO2, and 0% to 10.0% of PbO by weight, where the coefficient of linear expansion in a temperature range of 30° C. to 380° C. is 43×10−7/° C. to 55×10−7/° C.
Japanese Patent No. 3786397 describes a glass tube for a fluorescent lamp using glass containing substantially no PbO but containing 55% to 75% of SiO2, 1% to 10% of Al2O3, 10% to 25% of B2O3, 0.01% to 3% of ZrO2, 5% to 15% of Li2O+Na2O+K2O and 0.1% to 10% of at least two compounds from among WO3, Sb2O3, Nb2O5, Bi2O3, CeO2 and Fe2O3 by mass, where the average coefficient of linear expansion in a temperature range of 50° C. to the glass transition point (Tg) is 48×10−7/° C. to 58×10−7/° C.
In addition, Japanese Unexamined Patent Publication 2007-210851 describes a glass tube for a fluorescent lamp made of borosilicate based glass containing 55% to 75% of SiO2, 1% to 7% of Al2O3, 10% to 25% of B2O3, 0.1% to 3% of Li2O, 3% to 9.5% of K2O, 0.1% to 5% of CaO+MgO+BaO+SrO, 0.01% to 5% of Nb2O5+SnO2+ZrO2+ZnO, 0.1% to 5% of CeO2+WO3+V2O5 and 0.001% to 0.05% of Fe2O3, where the average coefficient of linear expansion in a temperature range of 0° C. to 300° C. as defined in JIS-R-3102 is 36×10−7/° C. to 57×10−7/° C.
However, these gazettes do not describe any optimal amount for the added elements, nor any ratio for rare earth elements to be contained.